This invention relates generally to processes and systems for removing hydrogen sulfide from a gaseous stream. More specifically the invention relates to improvements in a known process and system wherein hydrogen sulfide is removed from a gaseous stream, using a nonaqueous scrubbing liquor in which are dissolved sulfur and a reaction-promoting amine base. In a first aspect of the invention sulfur dioxide is added as an oxidizing gas to the sulfur-amine nonaqueous sorbent (or advantage is taken of SO2 which may already be present in the gas stream) to obtain better H2S removal, lower chemical degradation rates, and lower rates of formation of byproduct sulfur salts. In a further aspect of the invention the gas to be treated is mixed with oxygen and passed through an oxidation catalyst reactor to either effect oxidation of part of the H2S to form the required amount SO2 for reaction with the remaining H2S, or to effect partial oxidation of the H2S in the feed gas to form elemental sulfur, or to form various combinations of products as desired for the application, prior to scrubbing with the nonaqueous solvent.
Conventional liquid redox sulfur recovery processes use a redox couple dissolved in water to scrub hydrogen sulfide from a gas stream and convert it to sulfur. The redox agent is reduced by the hydrogen sulfide and then is regenerated by contacting with air in a separate vessel. One of the main problems with such processes is dealing with the solid sulfur product, which is formed in an uncontrolled manner. The sulfur formed from aqueous solution is notorious for plugging the absorber or other vessels which is passes through, and it is generally hard to separate and handle. Sulfur formed from nonaqueous solvents has much better handling properties. However, most nonaqueous redox systems have certain disadvantages such as sluggish sulfur formation kinetics or difficulties in regenerating the sorbent with air. In aqueous systems, contact of polysulfides with air primarily produces sulfates and other undesired sulfur oxyanion byproducts which are difficult to purge from the system.
The present inventor""s U.S. Pat. No. 5,738,834, the entire disclosure of which is hereby incorporated by reference, discloses a process which uses a sulfur-amine nonaqueous sorbent (SANS) and operating conditions under which sulfur itself can convert hydrogen sulfide to polysulfides which are nonvolatile but which can be readily transformed to sulfur by reaction with an oxidizing agent. This is done in a solvent with a high solubility for sulfur so that solid sulfur formation does not occur in the absorber or in the air-sparged regenerator. Solid sulfur formation can be initiated in process equipment designed to handle solids and can be done under well-controlled conditions. In the SANS process, the sour gas is fed to an absorber (typically countercurrent) where the H2S is removed from the gas by a nonaqueous liquid sorbing liquor which comprises an organic solvent for elemental sulfur, dissolved elemental sulfur, an organic base which drives the reaction converting H2S sorbed by the liquor to a nonvolatile polysulfide which is soluble in the sorbing liquor, and an organic solubilizing agent which prevents the formation of polysulfide oilxe2x80x94which can tend to separate into a separate viscous liquid layer if allowed to form. The solubilizing agent is typically selected from the group consisting of aromatic alcohols and ethers including alkylarylpolyether alcohol, benzyl alcohol, phenethyl alcohol, 1-phenoxy-2-propanol, 2-phenoxyethanol, alkyl ethers including tri(propylene glycol) butyl ether, tri(propylene glycol) methyl ether, di(ethylene glycol) methyl ether, tri(ethylene glycol) dimethyl ether, benzhydrol, glycols such as tri(ethylene) glycol, and other polar organic compounds including sulfolane, propylene carbonte, and tributyl phosphate, and mixtures thereof. The sorbing liquor is preferably essentially water insoluble as this offers advantages where water may be condensed in the process. It is also preferable for water to be essentially insoluble in the solvent. The nonaqueous solvent is typically selected from the group consisting of alkyl-substituted naphthalenes, diaryl alkanes including phenylxylyl ethanes such as phenyl-o-xylylethane, phenyl tolyl ethanes, phenyl naphthyl ethanes, phenyl aryl alkanes, dibenzyl ether, diphenyl ether, partially hydrogenated terphenyls, partially hydrogenated diphenyl ethanes, partially hydrogenated naphthalenes, and mixtures thereof. In order to obtain a measurable conversion of sulfur and hydrogen sulfide to polysulfides, the base added to the solvent must be sufficiently strong and have sufficient concentration to drive the reaction of sulfur and hydrogen sulfide to form polysulfides. Most tertiary amines are suitable bases for this use. More particularly, tertiary amines including N,N imethyloctylamine, N,N dimethyldecylamine, N,N dimethyldodecylamine, N,N dimethyltetradecylamine, N,N dimethylhexadecylamine, N-methyldicyclohexylamine, tri-n-butylamine, tetrabutylhexamethylenediamine, N-ethylpiperidine hexyl ether, 1-piperidineethanol, N-methyldiethanolamine, 2-(dibutylamino)ethanol, and mixtures thereof are suitable for use in the said process. It should be noted that while the solvent utilized in the process requires the addition of a base to promote the reaction of sulfur and hydrogen sulfide to form polysulfides, the base and the solvent may be the same compound.
As it is removed, the H2S thus reacts with elemental sulfur and a tertiary amine, both dissolved in the sorbent, to form an amine polysulfide. One of the polysulfide-formation reactions in the absorber may be depicted as follows (where B stands for the amine, HB+ is the protonated amine, g denotes the gas phase, and l denotes the liquid phase).
H2S(g)+S8(l)+2 B(l)⇄(HB)2S9(l) xe2x80x83xe2x80x83(1) 
The stoichiometry shown in this equation is representative, although polysulfides of other chain lengths may be formed, and varying degrees of association of the amine and polysulfide may occur, depending on the specific solvent chemistry and operating conditions. The primary solvent is selected to have a high solubility for sulfur (as well as for the amine) so that the sorbent circulation rates can be low, producing small equipment sizes for both the H2S absorber and the solution regenerator. Another ingredient is normally added to the sorbent to solubilize the amine polysulfides which might otherwise separate. The sweet gas from the absorber exits the process. The rich sorbent from the absorber may be passed through a reactor to allow further time for polysulfide formation reactions to occur, if desired. The sorbent is flashed down to near atmospheric pressure in one or more stages, producing a small flash gas stream that can either be recycled or used as fuel for local power generation. The sorbent is then contacted with an oxidizing gas such as air in the regenerator to oxidize the polysulfide to elemental sulfur, which remains dissolved in the solvent. This reaction, which also frees the amine for the next sorption cycle, can be depicted as follows.
(HB)2S9(l)+xc2xd O2(g)⇄S8(l)xe2x88x92H2O(g)+2 B(l) xe2x80x83xe2x80x83(2) 
Under the proper chemical and physical conditions, the efficiencies for simple air regeneration are unexpectedly high and the rates of the air oxidation reaction to sulfur are unexpectedly fast for a nonaqueous system. Tertiary amines produce high regeneration efficiencies. Spent air from the oxidizer contains the product water. The sorbent stream from the oxidizer is cooled in a heat exchanger and fed to the crystallizer where the cooling causes the formation of crystalline sulfur. The sorbent is cooled to a sufficiently low temperature to crystallize enough solid sulfur to balance the amount of hydrogen sulfide absorbed in he absorber. This produces the same overall reaction as in other liquid redox sulfur recovery process.
H2S(g)+xc2xd O2⇄xe2x85x9 S8(s)+H2O(g) xe2x80x83xe2x80x83(3) 
The solvent generally can have a solubility for sulfur in the range of from about 0.05 to 2.5, and in some instances as high as 3.0 g-moles of sulfur per liter of solution. The temperature of the nonaqueous solvent material is preferably in the range of about 15xc2x0 C. to 70xc2x0 C. Sulfur formation is obtained, when desired, by cooling the liquor proceeding from the air-sparged regenerator. This can for example be effected at a sulfur recovery station by cooling means present at the station. The solvent is thereby cooled to a sufficiently low temperature to crystallize enough solid sulfur to balance the amount of hydrogen sulfide absorbed in the absorber. The solubility of elemental sulfur increases with increasing temperature in many organic solvents. The rate of change of solubility with temperature is similar for many solvents, but the absolute solubility of sulfur varies greatly from solvent to solvent. The temperature change necessary to operate the process will vary primarily with the composition of the sorbent, the flow rate of sorbent, and the operating characteristics of the recovery station. For most applications, a temperature difference of 5xc2x0 C. to 20xc2x0 C. is appropriate as between the temperature of the solvent material at the absorber/reactor and temperature to which the said solvent is cooled at the sulfur recovery station; The regenerated sorbent from the crystallizer is recycled back to the absorber. The slurry of crystalline sulfur from the crystallizer is thickened and fed to a filter that produces a filter cake of elemental sulfur for disposal or sale.
The reaction between sulfur and H2S to form polysulfide is chemically reversible and the physical solubility of H2S in the sorbent is high. The equilibria are such that at low inlet H2S concentrations it becomes difficult to achieve high H2S removals at acceptably low liquid flow rates due to the xe2x80x9cback-pressurexe2x80x9d of H2S.
Now in accordance with a first aspect of the present invention, it has been found that in the process of the 5,738,834 patent, the addition of SO2 to the absorber produces a more complete chemical conversion of H2S thus reducing the equilibrium back-pressure of H2S and allowing much better removals to be obtained. The SO2 is thereby used as an or the oxidizing gas referred to in the 5,738,834 patent. A major reaction appears to be between SO2 and the amine polysulfide formed in the initial SANS reaction:
2 (HB)2S9(l)+SO2⇄19/8 S8(l)+2 H2O(g)+4 B(l) xe2x80x83xe2x80x83(4) 
Combining this with equation 1 gives the overall reaction:
2 H2S(g)+SO2(g)⇄xe2x85x9c S8(l)+2 H2O(g) xe2x80x83xe2x80x83(5) 
This is the same as the well-known Claus reaction which is usually practiced in the gas phase at elevated temperatures using a catalyst. The gas phase Claus process is highly exothermic and equilibrium limited in the temperature range where it is normally practiced. Many attempts have been made at devising a liquid phase Claus reaction-based H2S removal process to circumvent the equilibrium limitation. Most of these attempts have been plagued by the sluggish kinetics of the direct low temperature reaction of H2S and SO2 and by formation of intractable mixtures of polythionates (Wackenroder""s solution) and other undesirable byproducts. Use in the present invention of a hydrophobic solvent having a high sulfur dissolving ability removes these defects. The high concentration of sulfur promotes the formation of polysulfide when reacted with H2S and the polysulfide reacts rapidly and completely with SO2.
The process can be operated with an excess of H2S over the classical xe2x80x9cClausxe2x80x9d mole ratio of 2 mole H2S per mole SO2 where air is used as an oxidant in addition to SO2. This is illustrated by Example 1 (see below) which used an input mole ratio of 3 mole H2S per mole SO2. Under these conditions, reactions (2) and (4) apparently operate in parallel to form the product sulfur. During the 44 hour run under these conditions, the sulfur oxyanion byproduct make rate was only 1.92% (total moles sulfur in oxyanion byproducts per mole sulfur absorbed). This is somewhat lower than the value usually obtained in the SANS process in the absence of SO2, and illustrates that the conventional view that mixtures of sulfur and SO2 necessarily produce a lot of thiosulfate does not apply to the process conditions described here. A significantly lower than normal amine degradation rate of 0.28% (mole amine degraded per mole sulfur absorbed) was also observed in this run. This is to be compared with the value of about 1.0% (mole amine degraded per mole sulfur absorbed) previously observed in runs done with no SO2. This increase in amine stability could be due to the oxidation of polysulfide with SO2 being less harsh and generating less reactive oxidizing intermediates than the oxidation of polysulfide with oxygen.
When operated without any air regeneration, the SO2-enhanced SANS process operates at a nominal H2S:SO2 mole ratio of 2:1 in accordance with the usual overall Claus reaction. However the sorbent composition, particularly the high elemental sulfur concentration, provides a buffering effect which allows extended operation under xe2x80x9coff-ratioxe2x80x9d inlet gas conditions. This is important for obtaining stable operation and for meeting environmental regulations which may limit short term peak emissions. During episodes of higher than stoichiometric addition of SO2, the high elemental sulfur concentration in the solution allows the formation of thiosulfate.
SO2(l)+xe2x85x9 S8(l)+2 B(l)+H2Oxe2x86x92(HB)2 SO2O3(l) xe2x80x83xe2x80x83(6) 
which can later be converted to product sulfur by reaction with H2S or a polysulfide.
(HB)2 S2O3(l)+2 H2S(l)xe2x86x92xc2xd S8(l)+2 B(l)+3 H2O xe2x80x83xe2x80x83(7) 
(HB)2 S2O3(l)+2 (HB)2S9(l)xe2x86x925/2 S8(l)+6 B(l)+3 H2O xe2x80x83xe2x80x83(8) 
Thus the elemental sulfur serves as a redox buffer for both of the main reactants, reacting either with excess H2S to form polysulfide, or with excess SO2 to form thiosulfate. Importantly, the products of these reactions can then be efficiently converted to elemental sulfur when the H2S:SO2 ratio swings back the other way, and byproduct formation is minimal.
The process of the foregoing aspect of the present invention can be applied to many different sulfur recovery situations with the addition of SO2 being accomplished by several possible means including (1) obtaining SO2 as a gas or liquid from an independent source and injecting it into the inlet gas or absorber; (2) burning product sulfur and injecting the resultant SO2 as in the (1); and (3) converting a portion of the H2S or other sulfur species in the inlet gas to SO2 by passing a portion of the inlet gas stream along with air or oxygen through a catalyst bed or other device which will provide the desired amount of SO2.
To summarize, one major advantage of the aforementioned aspect of the invention is that better H2S removal can be obtained if SO2 is present in the scrubbing liquor or in the inlet due to the more complete chemical reaction of H2S in the absorber. Another advantage is that the H2S can conveniently be converted to elemental sulfur in the absorbing vessel, which may be at high pressure, thus minimizing the flashing of H2S when the liquid is xe2x80x9cflashedxe2x80x9d down to the lower pressure. This property of conversion to sulfur in one vessel also provides the opportunity to maintain the circulating liquid at operating pressure, thus eliminating the flashing of volatile hydrocarbons, as can happen if the liquid pressure is reduced prior to regeneration in an oxidizer operating near ambient pressure. Yet another advantage is the decrease in degradation of amine, apparently due to the less harsh oxidant conditions provided by SO2 compared to oxygen.
In a further aspect of the invention the gas to be treated is mixed with oxygen and passed through an oxidation catalyst reactor to either effect oxidation of part of the H2S to form the required amount SO2 for reaction with the remaining H2S, or to effect partial oxidation of the H2S in the feed gas to form elemental sulfur, or to form various combinations of products as desired for the application, prior to scrubbing with the nonaqueous solvent.